1. Field of the Invention
The present invention relates to a process for producing a polyamide from a dicarboxylic acid and a diamine. More particularly, the invention relates to a process with improved production efficiency for producing a polyamide by adding a diamine to a molten dicarboxylic acid in the absence of a solvent to conduct a direct polymerization therebetween.
2. Description of the Prior Art
Polyamide has been generally produced using, as a raw material, a nylon salt or an aqueous solution thereof. The process for the production of polyamide may be carried out in either a batch-wise manner or a continuous manner using such a raw material. For example, in the batch-wise method, an aqueous solution of nylon salt is heated under pressure in one reactor where the polymerization of the nylon salt proceeds in a uniform phase while inhibiting the diamine component from being lost by distillation. Then, after fixing the diamine component by polymerization, a steam in the reaction system is gradually released, and the polymerization reaction is completed finally under ordinary or reduced pressure. In the continuous method, while continuously feeding the raw material, the same steps as in the batch-wise method are sequentially performed, and the polyamide as produced is continuously discharged.
However, the use of the aqueous solution nylon salt as the raw material causes the following problems. The aqueous solution of nylon salt usually has a concentration of about 50%. The presence of a large amount of water requires a highly pressure-resistant reactor because a high pressure is needed to prevent water as the solvent from being distilled off in an initial stage of the polymerization. In addition, a large amount of water used as the solvent must be finally removed together with condensation water generated during the polymerization. Upon the removal of water, there arise various inconveniences such as foaming, solidification of polyamide due to a latent heat of water vaporization, and heat degradation of polyamide which is remained on a wall of the reactor due to a large change of the liquid level during the reaction. Therefore, a counter measure should be laid down against these inconveniences. In addition, the process using a aqueous solution of nylon salt not only requires a large amount of heat energy for the removal of water, but also involves many technical or economical problems such as low polyamide yields per one batch operation.
In the process where a nylon salt is used as the raw material as described in Japanese Patent Publication No 33-15700 and No. 43-22874, many of the above problems are avoided, but additional steps for separation and purification of the nylon salt are required, thereby failing to provide an efficient process for the production of polyamide.
As a polymerization process using a raw material other than the nylon salt and the aqueous solution thereof, there are known a method in which a diamine containing a small amount of water is added dropwise into a dicarboxylic acid at 220xc2x0 C. or lower under ordinary pressure (Japanese Patent Application Laid-Open No. 48-12390); a method in which a diamine is added dropwise into a molten dicarboxylic acid under ordinary pressure to directly conduct the reaction therebetween (Japanese Patent Application Laid-Open No. 57-200420 and No. 58-111829). These methods are advantageous both technically and economically as compared to those using the nylon salt and the aqueous solution thereof. However, the methods must be practically performed in a batch-wise manner, and it is considerably difficult to perform these methods using a continuous-type polymerizer.
When an apparatus for the production of polyamide is designed, care must be paid to heat supply, surface renewal capability, uniform stirring and mixing, or the like. In order to ensure the same production yield of polyamide between a batch-wise polymerization and a continuous polymerization, the size of the apparatus is limited lower in the batch-wise polymerization as compared with the continuous polymerization. Further, in the batch-wise polymerization, the polyamide is discharged from the reactor preferably within one hour after the molecular weight reaches a predetermined value. Therefore, a pelletizer for polyamide is required to have a capacity capable of treating one batch of polyamide within one hour. With recent rapidly-increasing throughput capacity of pelletizer, a considerably large scale of pelletizer has come to be commercially available. Nevertheless, the capacity is still limited to 10 tons per hour at most. Therefore, in view of the capacity of pelletizer, the maximum production of polyamide in the batch-wise process is limited to 10 tons per one batch. If a larger production is intended, it is necessary to use a plurality of pelletizer, resulting in extremely poor production efficiency. In a method, polyamide produced by the batch-wise process is temporarily stored in a molten state, and then gradually discharged into to a pelletizer where the polyamide is continuously pelletized. This method enables the use of a pelletizer having a small throughput capacity. However, in this method, there arise problems such as change in polymerization degree of the polyamide during storage, break of strands due to inclusion of air bubbles, thereby failing to perform a continuous pelletization.
The batch-wise polymerizer and the continuous polymerizer each have respective merits and demerits, and therefore, the superiority thereof is not simply determined. In general, the continuous polymerizer is suitable for a small-kind and large-quantity production, while the batch-wise polymerizer is suitable for a multikind and small-quantity production. The turning point where the continuous process becomes economically advantageous over the batch-wise process is said to be 10,000 tons or 20,000 tons by annual production. In a production process suitably performed by a batch-wise method or in a batch-wise process where a production amount exceeding the range advantageous for the batch-wise production is intended, it is important to consider how the production efficiency of the batch-wise process is enhanced. Thus, it has been demanded to develop a more efficient process for the production of polyamide.
An object of the present invention is to provide a process for producing a polyamide with an enhanced production efficiency by adding a diamine to a molten dicarboxylic acid in the absence of a solvent to conduct a direct polycondensation therebetween.
As a result of extensive researches in view of the above object, the inventors have found that, in the process for producing a polyamide by adding a diamine to a molten dicarboxylic acid to conduct a direct polycondensation therebetween in the absence of a solvent, the polymerization process is smoothly switched from a batch-wise manner to a continuous manner and the production efficiency is enhanced by allowing the polycondensation to proceed up to a middle stage in a batch-wise first polymerizer, filling a vapor-phase portion contacting the middle stage polyamide with steam having a predetermined pressure, and then feeding the polyamide to a continuous second polymerizer while maintaining the polymerization degree of the middle stage polyamide substantially constant. The present invention has been accomplished on the basis of this finding.
Thus, in a first aspect of the present invention, there is provided a A process for producing a polyamide by melt-polycondensing a diamine with a dicarboxylic acid in the absence of a solvent, comprising (1) a step of melting the dicarboxylic acid; (2) a step of continuously or intermittently adding the diamine into the dicarboxylic acid kept in a molten state in a batch-wise first polymerizer equipped with a partial condenser to subject the diamine and the dicarboxylic acid to polycondensation at a predetermined molar ratio to produce a middle-stage polyamide having a relative viscosity of 1.4 to 2.7, the diamine having a boiling point higher than a melting point of the middle-stage polyamide under an inner pressure of the first polymerizer; (3) a step of feeding the middle-stage polyamide from the first polymerizer to a melt retention tank, where the middle-stage polyamide is fed into a continuous second polymerizer while controlling a change of relative viscosity of the middle-stage polyamide during the retention in the melt retention tank within xc2x10.2 by maintaining a vapor phase portion of the melt retention tank at a predetermined pressure by saturated steam; and (4) a step of subjecting the middle-stage polyamide in the second polymerizer to a further polycondensation while removing water from a molten reaction system under reduced pressure, thereby producing a later-stage polyamide.
In a second aspect of the present invention, there is provided a process for producing a polyamide by melt-polycondensing a diamine with a dicarboxylic acid in the absence of a solvent, comprising (1) a step of melting the dicarboxylic acid; (2) a step of continuously or intermittently adding the diamine into the dicarboxylic acid kept in a molten state in a batch-wise first polymerizer equipped with a partial condenser to subject the diamine and the dicarboxylic acid to polycondensation at a predetermined molar ratio to produce a middle-stage polyamide having a relative viscosity of 1.4 to 2.7, the diamine having a boiling point higher than a melting point of the middle-stage polyamide under an inner pressure of the first polymerizer; (3) a step of feeding the middle-stage polyamide from the first polymerizer into a continuous second polymerizer while controlling a change of relative viscosity of the middle-stage polyamide in the first polymerizer within xc2x10.2 by maintaining a vapor phase portion of the first polymerizer at a predetermined pressure by saturated steam; and (4) a step of subjecting the middle-stage polyamide in the second polymerizer to a further polycondensation while removing water from a molten reaction system under reduced pressure, thereby producing a later-stage polyamide.